A conventional cooling method in vehicle air-conditioning systems is a vapor compression method. As shown in FIG. 9, a refrigeration cycle in a vapor compression method is a closed circuit constituted by a compressor 100, a condenser 101, an expansion valve 102, and an evaporator 103. A refrigerant is circulated through the closed circuit to cool a passenger compartment, utilizing heat of evaporation of the refrigerant. This principle will be described along the flow of the refrigerant.
A liquefied refrigerant A passes through the expansion valve 102, adiabatically expanding to a reduced pressure and temperature, and in the form of a spray B, enters the evaporator 103, absorbing heat from air in the passenger compartment while evaporating, and in the form of a heated vapor C, is sucked into the compressor 100, adiabatically compressed, and in the form of a high-temperature high-pressure gas D, reaches the condenser 101, releasing heat outside, and returns to the initial state A.
In the above-described refrigeration cycle, to prevent the evaporator 103 from freezing, the compressor 100 is turned off when the evaporator outlet air temperature is lower than or equal to a minimum temperature, e.g., 4° C. For power saving of the compressor 100, the target temperature of evaporator outlet air at which to turn off the compressor 100 is set at a predetermined temperature greater than or equal to the minimum temperature, based on an outside air temperature and a target discharge temperature. The compressor 100 is turned off when the evaporator outlet air temperature is less than or equal to the predetermined temperature (see, e.g., Japanese Patent Laid-Open Publication Nos. SHO-62-25522 and SHO-63-17642).
After turned off, the compressor 100 is again turned on when the evaporator outlet air temperature rises 1° C. from the predetermined temperature, for example.
In an air-conditioning system disclosed in Japanese Patent Laid-Open Publication No. SHO-61-20693, when the compressor outlet refrigerant pressure is greater than or equal to a predetermined pressure, the speed of a condenser fan is switched from low speed to high speed because a condenser requires a relatively large amount of cooling air.
However, in a vehicle air-conditioning system having the above-described two characteristics, when the predetermined temperature is high, as compared to the case where it is low, the on-off operation of the compressor is repeated under high outlet pressure, increasing the rising speed of the outlet pressure. Consequently, the compressor outlet refrigerant pressure reaches the predetermined pressure in a shortened period of time after the compressor is turned on, and the condenser fan is switched from low speed to high speed at a relatively early stage. The occurrence of this phenomenon with a vehicle stopped causes the problem that passengers are likely to perceive it as noise. It is desirable to make it hardly perceived as noise.